One example of conventional NMR diagnostic apparatus is described in U.S. Pat. No. 4,254,778.
First, a static magnetic field is generated by four electromagnetic coils C1 shown in FIGS. 1 (a) and (b), and a gradient magnetic field generated by gradient magnetic field coils C2, C3 and C4 shown in FIGS. 2 and 3 is superimposed on the static field. Coils C2, C3 and C4 are installed to be combined with coils C1.
Such a situation is schematically illustrated in FIG. 4. Initially, the static field Hzo due to coils C1 is applied to the object with the gradient field Gz due to coils C2. The gradient field Gz is established by flowing current in the reverse direction in a pair of coils of the Helmholtz type shown in FIG. 2. Therefore, the direction of its magnetic force lines is the same (the Z-direction) as that of the static field Hzo. Also, the strength of the gradient field is zero at the center plane between the two coils C2, and its absolute value increases linearly from the center plane to either side thereof, but the direction of the field strength is opposite one another.
Next, when a selective exciting pulse H1 having an appropriate frequency component is applied to the composite magnetic field through a pair of probe-head coils C5 shown in FIG. 5, resonance occurs only in a plane in which a resonance frequency determined by the local field due to the fields Hzo and GZ is equivalent to the frequency of the pulse H1. To the plane of resonance, a gradient magnetic field GR formed by a combination of the magnetic fields GX and GY is applied. Measured through coils C5 is a free induction decay (FID) signal which is Fourier transformed to obtain the projection data of density distribution of atomic nuclei, for example, hydrogen along the gradient field GR in the selected plane. If projection data along a variety of directions is obtained by shifting the gradient field GR direction, the hydrogen nuclei density distribution image in a slice of the object is reconstructed using well-known techniques of X-ray computed tomography.
On the other hand, the multi-sensitive point imaging technique is known as one technique which does not require the above-mentioned image reconstruction as in Hinshaw, Journal of Applied Physics, Vol. 47, No. 8, August 1976. Such a technique operates on the principle that the gradient magnetic field is oscillated by flowing an alternate current into the gradient field coils and in the meantime FID signals are continuously measured for integration, resulting in signals only on the center line in which the gradient field is not changed with time.
There are many cases in which it is difficult to perform precise diagnosis only with the information obtained by NMR diagnostic apparatus using recent medical diagnostic techniques. Therefore, in such cases it is necessary to perform diagnosis in association with an X-ray image, X-ray CT image, ultrasonic image, nuclear medicine image or the like.
It is insufficient to utilize only tomographic images due to the NMR apparatus, which images are those perpendicular to the body axis of the human subject. One-directional projection images which are those projected in the one direction, are needed for positioning in correspondence to other imaging means, for example, X-ray images utilized in a variety of fields. Further, it could be easily supposed that not only such a positioning but also an examination is established with the one-directional projection image because general X-ray radiographic images are used popularly for examination. Although the formation of tomographic images is being pursued in the present NMR techniques while acquiring one-directional projection images, it is not sufficiently developed. This is especially true using a method in which an object is mechanically shifted like one performed using a conventional X-ray CT apparatus, which has the following disadvantages: (a) measuring time is long; (b) it is insufficient if the subject suffers pain due to movement; (c) the blur of images due to the mechanical operation; and (d) even during measurement, if the object is shifted, blur is caused in images.